A steel material for automobile structural use or machine structural use is required to exhibit excellent mechanical properties such as strength, formability, toughness and the like. Since these mechanical properties can be effectively improved by refining the grains of the material structure, various methods for producing a material having fine grain structure are being investigated. In the field of high tensile strength steel sheets, in particular, there are intensive needs for steel sheet which is capable of reducing the production cost and exhibiting excellent functional properties. Thus, the target of research and development has been shifted to steel sheet which satisfies the above-mentioned needs. In order to restrain deterioration of ductility, toughness, endurance ratio or the like which may arise from increased tensile strength, it is important to refine the structure of high tensile strength steel. Furthermore, in the field of cold rolled steel sheets for automobile use or the like, it is recognized that refining the structure of the hot rolled steel sheet as the raw material effectively improves the formability, especially the "r-value" or so-called Lankford value. Thus, refining the structure of hot rolled steel sheet is also important particularly when it is used as the raw material for cold rolled steel sheet.
Conventional measures for refining the structure of the materials can be classified into large reduction rolling method, controlled rolling method, controlled cooling method and the like. Among others, a large reduction rolling method for refining the material structure is proposed, for example, in JP-A-58-123823. The refining mechanism of the large reduction rolling method is to promote strain induced transformation from .gamma. phase to .alpha. phase due to an increased reduction on austenite grains of the material. While the known method achieves a certain degree of refining, there is a problem associated with the production technology that it is difficult to carry out with general hot strip mills since, for example, not less than 40% of rolling reduction per one pass is needed. Moreover, the refining of the obtained final structure is limited due to the product conditions which are difficult to realize, so that the average grain diameter of the final structure cannot be reduced to less than about 5 .mu.m. Further, the obtained grains are compressed and flattened due to large reduction rolling, thereby giving rise to problems that anisotropy of mechanical properties becomes significant or fracture-absorbed energy is decreased as a result of so-called separation or delamination.
On the other hand, there is known a precipitation strengthening steel sheet comprising Nb or Ti, as a steel sheet which has been subjected to refining by the controlled rolling method or controlled cooling method. The precipitation strengthening steel sheet is strengthened by utilizing the precipitation strengthening action of Nb or Ti, and has ferrite grains which have been refined by utilizing the austenite grains recrystallization inhibition action of Nb or Ti, and also by strain induced transformation to .alpha. phase from .gamma. phase of the anrecrystallized deformed austenite grains in finish rolling under a low temperature condition. However, the precipitation strengthening steel sheet has a problem that it has a large anisotropy of mechanical properties. For example, when the steel sheets having a large anisotropy of mechanical properties is applied for automobile use and subjected to press forming process, the effects of the refined structure may not be fully apparent because the forming limit of the material is limited to the property level in the direction of the worst ductile property. This is also the case when the precipitation strengthening material is used for structural materials, wherein the effects of the refined structure may not be fully apparent because the steel sheet has a large anisotropy of toughness or fatigue strength, which are important properties for structural materials. Moreover, the grain diameter of the structure subjected to such refining method as the controlled rolling method or controlled cooling method cannot be reduced to below about 2 .mu.m.
Furthermore, it is known to inhibit the grain growth of the material by rapid cooling immediately after hot rolling (refer, for example, to JP-B-4-11608), though the grain diameter of the structure obtained by such method cannot be reduced to below about 4 .mu.m.
As mentioned above, the grain diameter of the structure of the material which can be achieved by the prior art is limited to 2 .mu.m. In general, the effect of improvement in the mechanical properties by refining the grains is in inverse proportion to a square root of grain diameter. Therefore, while little improvement can be achieved when the grain diameter is not less than 2 .mu.m, a considerable improvement can be achieved if the grain diameter can be successfully reduced to below 2 .mu.m.